The Rambam (Hil. Shabbos 17:12-13) rules that a lechi may be made from a forbidden asheirah tree, but not a korah. The Ra'avad(ad loc.) asks why there is a difference: Asheirah wood cannot serve as a korah, because a korah needs to possess a minimum measurement (of a tefach). Since asheirah wood is כתותי מכתת שיעוריה — i.e. since it must be burnt, it is considered as if already burnt, and therefore is treated as ashes — it cannot meet the required measurement of a tefach. However, a lechi also needs to possess a minimum measurement — a height of ten tefachim. If so, asks the Ra'avad, why is a lechi of asheirah acceptable?

R' Chaim Soloveitchik of Brisk (ad loc.) addresses the question. He explains that the Rambam's opinion is that the required measurement of ten tefachim of height does not pertain to the walls of a reshus ha'yachid, but to the enclosed area — viz., an area that is ten tefachim in height must be surrounded by walls. In theory, once an area that is ten tefachim high is enclosed by walls, it is a reshus ha'yachid regardless of whether the walls that enclose it are ten tefachim high or not.

A lechi (although not necessarily a korah) functions like a fourth wall of a mavoi. Hence, it has the same parameters of a wall. Thus, so long as it "encloses" a height of ten tefachim, it creates the necessary condition for the enclosed area to be considered a reshus ha'yachid. It does not matter that as a lechi of asheirah the lechi itself does not posses the minimum measurement. This is distinct from a korah, in which case the Rabbis required that the korahitself possess the minimum measurement of a tefach.